Continuous precision tuning system



March 18, 1958 J. R. WHITE CONTINUOUS PRECISION TUNING SYSTEM Filed Jan.24, 1955 INVENTOR Jn/v R. Wfl/Te Iv... "su: S Q

H7702' EY United States Patent 2,827,567. Patented Mar. 18, 1958 hiceCONTINUOUS PnnclsroN TUNrNG SYSTEM John R. White, Westbury, N. Y.,assigner, by mesne assignments, to the United States of America asrepresented by the Secretary of the Navy Application January 24, 1955,Serial No. 483,347

1 Claim. (Cl. 250-35) This invention concerns the continuous precisiontuning of an oscillator and a system therefor.

Oscillator tuning methods of the prior art include those wherein relatedharmonics or nonharmonics are added to the frequencies produced bydifferent crystal oscillators and the undesirable harmonics are filteredout. The ltering out of undesirable harmonics in these methods isextremely difficult and requires many racks of equipment. Frequencyadding methods produce results which contain many spurious frequencies.

Other prior art methods include the precision calibration of theoscillator to be tuned. This calibration will drift over a period oftime, so that the oscillator has to be recalibrated periodically.

The continuous precision tuning system of the present invention operatesin a manner which may be assimilated to an automatic telephone switchingsystem operating in reverse. That is, the oscillator to be tuned is rstroughly tuned to a frequency below the lower frequency of its band, andthen frequencies are added to said lower band frequency until the exacttuning frequency is reached.

The principal object of this invention is the provision of a system forcontinuously tuning an oscillator in a precise and rapid manner.

An object of this invention is the provision of a continuous tuningsystem which tunes an oscillator' precisely to any desired frequency, toany desired degree of precision within the practical limitations of thecomponents of the system. y

Another object of this invention is the provision of a precision tuningsystem which tunes an oscillator without producing undesirable spuriousfrequencies.

A further object of this invention is the provision of a system for thecontinuous precision tuning of an oscillator in which the tunedfrequency of said oscillator does not drift.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawing wherein the ligure is thecontinuous precision tuning system of the present invention in blockdiagram form.

The oscillator to be tuned by the system of the figure is the masteroscillator 4.

Electrical digital intelligence, containing the ordered tuning frequencydata for the oscillator 4, conducted through the electrically conductinginput line 12 is received from either a remote source or from a localsource. The electrical frequency data intelligence may be transmitted bywireless or by direct wire from a source of intelligence to the inputline 12 of the gure. If the electrical intelligence has a local sourceit may originate at a manually operated keyboard apparatus (not shown inthe figure).

In a preferred embodiment of the present invention, illustrated by thefigure, an electrical ordered frequency data signal in the form ofbinary digital intelligence is received in the line 12 either by wire orby radio transmission, or by manual operation of a keyboard whichoperates binary type lever switches. The received binary intelligencemay be in the form of carrier modulation or teletypewriter pulses.

The input switch 1i) may be manually closed upon receipt of an inputsignal in the input line 12 or it may be automatically closed upon thereceipt of an input signal. When the input switch lil is closed itcloses the input line 12 to the continuous tuner 5, to the frequencydata input switching unit l and to the band switch 2.

Upon the closing of the circuit between the input line 12 and thecontinuous tuner 5, by the input switch 10, the continuous tuner 5commences the operation of the system by tuning the master oscillator 4to a frequency below the lower frequency of the preselected band,wherein the frequency, at which the oscillator l isv to be tuned, lies.The continuous tuner 5 then automatically starts to tune across the bandcontaining the tuning frel quency.

A continuous tuner, which may be utilized, is a motor driven condenserwhich slowly tunes up a preselected band.

Upon the closing of the circuit between the input line l2 and the bandswitch 2, by the input switch 1G, the band switch 2 places theoscillator i and the band edge oscillator 7 in the preselected bandcontaining the tuning frequency, due to the energization of the bandswitch 2 by the input intelligence signal which indicates and identitiesthe band in which the tuning frequency of the master oscillator d islocated.

The band switch 2 is not necessary if the frequency to which the masteroscillator 4 is desired to be tuned remains in one preselected band andif the band edge oscillator 7 is selected according to such a band.However, if the Various tuning frequencies of the oscillator 4 varywidely enough to be located in dierent preselected bands, the bandswitch 2 is necessary for proper operational range. The band switch 2switches different oscillator coils into the circuit of the masteroscillator 4 and it switches crystals of diderent frequencies into thecircuit of the band edge oscillator 7 in accordance with the orderedtuning frequency of the oscillator 4 and the preselected band whereinthe tuning frequency lies. The band switch 2 thus permits operation overa wide range of tuning frequencies.

Upon the closing of the circuit between the input line 12 andthefrequency data input switching unit 1, by the input switch 10, thefrequency data input switching unit 1 operates to energize the propersequence of preci sion frequency generators 3 in accordance withyordered tuning frequency for the oscillator 4. The input switching unit1 energizes a sequence of precision fre-Y i quency generators 3, orderedby the input intelligence, which produces a total frequency, which whenadded to the output frequency of the band edge oscillator 7, produces asum total frequency equal to the ordered tuning frequency of the masteroscillator 4l. The upper frequency of the band minus the lower frequencyof the band must equal the sum of the output frequencies of theprecision frequency generators 3, to enable the system to work up frombelow the lower band frequency to any ordered tuning frequency of themaster oscillator 4 in the band.

The input switching unit may be either manually controlled or controlledby received intelligence.

The band switch and input switching circuits of the system are adjustedto be more rapid in operation than the continuous tuner circuit so thatthe previously described band switching and input switching operationsoccur before the continuous Ytuner has progressed upV the, precisionfrequency/generators 3, utilized in thersys-V tem of the ,ligure are twoto the zero power kilocycles,

two to the irst power kilocycles, two to the second power t kilocycles,two to the third power kilocycles,l two to the fourth power lrilocycles,twoto the fifth power kilocycles, two Vto the sixth power kilocycles,ytwo to the seventh power kilocycles, two to the eighth power kilocyclesand two tothe ninth 'power kilocycles. These precision frequencies arerespectively equal to 1 kilocycle, 2 kilocylces, 4-kilocycles, 8kilocycles, 16 kilocycles, 32 kilocycles,'64 kilocycles,l 128kilocycles, V256 kilocycles and 512 kilocycles. t

Y A very ne tuning'frequency may be obtained by utilizing precisionfrequency generators of low frequency outputs. v:The outputs may .beiselected in fractions of a kilocycle, such ask one half a kilocycle,.one fourthof a kilocycle, one eighth of a kilocycle; one sixtheenth ofa Vkilocycle,A one thirty second of a kilocycle, one sixty fourthv of akilocycle, one one hundred and twenty kilocycle, and soton. y

Y VThe basic vfrequency of the band edge oscillator 7 Vis controlledbythe band switch 2 in accordance with the input intelligence on theinput line 12. For single band operation there is one crystal in thecircuit of the hand edge oscillator 7. For multiband operation, there isa `crystal in the band edge oscillator circiut for each different bandin which the master oscillator is to be tuned.l However, the precisionfrequency generators 3 are rated similarly for all bands, since thepurpose of the band Yedge oscillator 7 is to translate the operation ofthe precision frequency generators 3 to any range of frequencies in anyband desired. This is due to the operation of the band edge oscillator 7throughyor without, the band switch 2 to attain a frequency output equalto the lower frequency of the band in which the tuning frequency of theoscillator'4 is located. The output frequencies of Ythe precisionfrequency generators 3 then build up'upon this lowerrband frequencyuntil the ordered tuning frequency Vis reached. The frequency which themixer 6 receives from the master oscillator 4 is a frequency below thelower freeighth of a quency of the band in which the tuning frequency oftheV master oscillator is located,V to which' the continuous tuner 5hasv brought it. TheY band edge oscillator 7 feeds the lowerfrequency'of the'band to the mixer 6 as does the master oscillator 4.The band edge oscillator 7 indicates when the master oscillator'4'hasreached the edgeV of the band in which its tuning frequency lies.

YThe master oscillator is anLC oscillator whose frequency is Yonlyapproximate and whose frequency may drift from time to time'. The'operation must therefore consistofrstarting at a reference point; afrequency below the wlower band frequencyin which thetuning frequencylies,V andV Working upto the tuning frequency each time the tuningoperation isperformed. Therefore, the master oscillator 4 does not haveto be a precision oscillator. The mixer 6 sends a` difference frequency,which is thedifference between the frequency fed 'to it by the masteroscillatorg4 and the frequency fedto it,by the the lowest frequency intheband inwhichthe master Y oscillator tuning frequency lies, the zerobeat indicator recognizes that fact'and produces a` pulse or `beat Vofcurrent; Y

The zero beat indicator 8 thus produces a beat or pulse which indicatesthat the master oscillator 4 is roughly tuned to the first of the digitsof the ordered tuning frequency; that is, the master oscillator 4 hasreached the t basic frequency of the band in which its tuning frequencylies. The zero beat indicator will not recognize the fact that themaster oscillator 4 is passing through the lower frequency of its bandunless it does pass through lsaid frequency. The zero beat indicator Sis merely a means for establishing the correspondence between theordered tuning frequency of the oscillator 4 andthe frequency to whichthe continuous tuner 5 has tuned the oscillator 4 at various intervalsin the tuning operation.V

The counting stepper 9 is a relay switching unit which is actuated bypulses received from the zero beat indicator S to close the appropriateprecision frequency generator circuit switches V11. The counting stepper9 steps itself down in sequence from the precision frequency generatorline with the highest frequency output rating to the precision frequencygenerator line with the lowest frequency output rating.

Upon receipt ofl an output pulse or beat from the zero beat indicator 8the counting stepper 9 commences its stepping operation .and stops atthe highest frequency rated precision frequency generator line which hasbeen energized by the frequency data input switching unitl. The countingstepper then operatesV to close the circuit switch 11 in the line atwhich it has stopped, to connect the output of the highest frequencyrated energized precision frequency generator 3 with the mixer. Thefrequency output of the selected `precision frequency generator isconducted to the mixer 6, where it is added toY the frequency output ofthe band edge oscillator l 'and the sum frequency is compared with thefrequency of the oscillator 4, which is the frequencytto which thecontinuous tuner 5 has tuned the oscillator 4 at such time. The zerobeat indicator 8 then produces a pulse 0r beat which is conducted to thecounting stepper 9 to continue the stepping operation and whichindicates that the'continuous tuner 5 has turned the oscillator 4 to afrequency which is the sum of the lower frequency of the tuningfrequency band and the output of the highest rated energized precisionfrequency generator.

When the beat, representing that the continuous tuner 5' has alreadytuned the oscillator 4 toa frequency equalV to the sum of thelowerfrequency of the tuning frequency band and the output of thehighest rated energized precision `frequency generator, is received bythe counting stepper 9, the counting stepper continues Vits steppingoperation Vand stops Yat the next highest frequency rated precisionfrequency generator line which has been ener gized bythe frequency datainput switching unit 1. The counting stepper then operates to close thecircuit switch 11 in the line at which it has stopped, to connect theoutput of the next highest frequency rated energized precision frequencygenerator 3 with the mixer 6, The frequency output of the selectedprecision frequency generator is conducted tothe mixer 6,l where it isadded to the frequency output of the band edgeV oscillator 7 plus theprior selected highest frequency generator output and the sum frequencyis compared with the-frequency of the oscillator 4, which is thefrequency to which the continuous tuner 5 has tuned the oscillator'4 atsuch time. The zero beat indicator 8 then produces la pulse or beatwhich is conducted to the counting stepper 9 to continue the steppingoperation and which indicates that the continuous tuner 5 has tuned-theoscillator 4to a frevquency Ywhich is the sum of the lower frequency of`the of the next'highest rated energized precision frequencyV generator.

tuning frequency band, the output 'of the highest rated energizedprecision frequency generator and the output When the beat or pulserepresenting that the continuous tuner 5 has already tuned theoscillator 4 to a frequency equal to the sumY of the lower frequency ofthe tuning frequency band, the output of the highest rated energizedprecision equency generator and the output of the next highest ratedenergized precision frequency generator, is received by the countingstepper 9, the counting stepper continues its stepping operation andstops at the precision frequency generator line having an output ratingequal to the next frequency to the next highest frequency ratedprecision frequency generator line which has been energized by thefrequency data input switching unit The stepping and continuous tunercontrol operations continue until all the energized precision frequencygenerator lines have been scanned by the counting stepper 9.

When the zero beat indicator 8 produces the beat or pulse whichrepresents that the continuous tuner 5 has tuned the oscillator 4 to itsordered tuning frequency, the counting stepper 9 steps past theremaining deenergized precision frequency generator lines until itreaches the end of its travel. The counting stepper 9 discovers that theordered tuning frequency has been reached when it finds that there areno longer any energized precision frequency generator lines awaiting itsaction. The counting stepper 9 then indicates to the continuous tuner Sthat the end of the operation has been reached by stopping thecontinuous tuner 5 precisely at the exact ordered tuning frequency.

The counting stepper 9 operates in accordance with standard steppingprocedure. The step-switching operation is designed for faster operationthan the continuous tuning operation, so that the continuous tuner willnot get ahead of the step-switching operation.

The input switch remains closed until the input switching unit 1 isturned off. When the input switch 10 is opened, either manually or byreceived intelligence from a remote source, the system is ready toreceive another input intelligence signal representing a new tuningfrequency for the master oscillator 4, even if such tuning frequency isin another preselected band. When the band switch 2 opens, at the end ofthe cycle of operation, the oscillator 4 is tuned at the frequencyordered by the input intelligence and all the circuits are sodeenergized that they :are ready for new infomation at any time thatsuch new information is received; the old information at such time iscompletely wiped out, although the oscillator 4 remains tuned.

When the input switch 10 closes again, the continuous tuner 5automatically drops back to its zero position, which is the lower end ofits tuning range. The system will then operate to tune the oscillator 4to the new ordered tuning frequency in a manner similar to thatdescribed.

The operation of the system of the present invention is most clearlypresented by describing the tuning of the master oscillator by anlarbitrarily selected ordered tuning frequency input intelligencesignal. On the assumpftion that the system receives input intelligenceordering it to tune the master oscillator to 3168 kilocycles theoperation is as follows.

Electrical digital intelligence, corresponding to the ordered tuningfrequency of 3168 kilocyclcs is received in the input line 12. The inputswitch 10 is closed to close the input line 12 to the ycontinuous tuner5, to the frequency data input switching unit 1 and to the band switch2.

Since the ordered tuning frequency is assumed to be 3168 kilocycles thepreselected band for this frequency may be assumed to be 3000 kilocyclesto 4000 kilocycles. Upon the closing of the circuit between the inputline 12 land the continuous tuner 5, by the input switch 10, thecontinuous tuner 5 commences the operation of the system by tuning themaster oscillator 4 to slightly below 3000 kilocycles. The continuoustuner 5 then automatically starts to tune across the 3000 to 4000kilocycle band.

Upon the closing of the circuit between the input line 12 and the bandswitch 2, by the input switch 10, the

bandV switch 2 places lthe oscillator 4 and the band edge oscillator 7in the 3000 to 4000 kilocycle band, due to the energization of the bandswitch 2 by the input intelligence signlal which indicates andidentifies the ordered tuning frequency band as being Ifrom 3000 to 4000kilocycles. The band switch 2 switches a 3000 kilocycle oscillator coilinto the ycircuit of the master oscillator 4 and it switches a 3000kilocycle crystal into the circuit of the band edge oscillator 7 inaccordance with the ordered tuning yfrequency and the preselected band.,

Upon the closing of the circuit between the input line 12 and thefrequency data input switching unit 1, by the input switch 10, thefrequency data input switching unit 1 operates to energize a sequence ofprecision frequency generators 3, ordered by the input intelligence,which produces a total frequency of 168 kilocycles.

The sum of the output frequencies of the precision frequency generators3 must equal 1000 kilocycles, which is the 4000 kilocycle upperfrequency of the band minus the 3000 kilocycle lower frequency of theband, to enable the system to work up from below 3000 kilocycles to yanyordered tuning frequency in the band. The precision frequency generators3 must therefore be selected and arranged to produce output frequenciesof 1, 2, 4, 8, 16, 32, 64, 128, 256 and 512 kilocycles corresponding toratings of the two (to the zero power, two to the first power, two tothe second power, two to fthe third power, two to the fourth power, twoto the fifth power, two to the sixth power, two to the seventh power,two to the eighth power and two to the ninth power kilocycles,respectively. The sum total of their frequency outputs being 1023kilocycles, the precision frequency generators 3 in Ythe system are ableto provide any ordered tuning frequency in any 1000 kilocycle band.

The input switching uni't 1, in order to provide the 168 kilocyclesnecessary to bring the lower 3000 kilocycle band frequency up to theordered 3168 kilocycle tuning frequency, energizes the 128 kilocyclerated precision frequency generator, the 32 kilocycle rated precisionfrequency generator and the 8 kilocycle rated precision frequencygenerator. This energization is facilely accomplished because the inputintelligence is in binary form zand because the output ratings of theprecision frequency generators are binary Valued. The precisionfrequency generators selected are respectively rated at two to theseventh power, two to the fth power and two 'to the third power.

The band switch and input switching circuits of the system are adjustedto be more rapid in operation than the continuous tuner circuit so thatthe previously described band switching and input switching operationsoccur before the continuous tuner 5 has progressed up into the 3000 to4000 kilocycle band.

The mixer 6 receives a frequency from the oscillator 4 which is below3000 k'ilocycles. The band edge oscillator 7 feeds 3000 kilocycles tothe mixer 6. The mixer 6 compares the output frequency of the oscillator4 with the output frequency of the band edge oscillator 7. When thecontinuous tuner 5 has tuned the oscillator 4 up to 3000 kilocycles, theoutput frequency of the oscillator 4 compares with the 3000 lc'locycleoutput frequency of the band edge oscillator 7 and the mixer 6 outputoperates the zero beat indicator to produce a beat 0r pulse whichindicates to the counting stepper 9 that the oscillator 4 is tuned to3000 kilocycles.

Upon receipt of the first output pulse or beat from the zero beatindicator 8 the counting stepper 9 commences its stepping operation bystepping past the deenergized 512 and 256 kilocycle lines and stops atthe 128 kilocycle output precision frequency generator line. Thecounting stepper then operates to close the circuit switch 11 in the 128kilocycle line to connect the 128 kilocycle precision frequencygenerator with the mixer 6. The mixer 6 adds the 128 kilocycles to the3000 kilocycle output of the band pass oscillator 7 and compares 'T7the-3128 kilocycl sum with the output frequency of the oscillator; 4.When VVthe Vcontinuous tuner tunes the oscillator 4 to y3128 kilocyclesthe mixer output operates the zero beatindicator 8 to produce a beat orpulse which indicates to the counting stepper 9 that the oscillator'4'istuned to 3128 kilocycles. Y

When the pulse produced by the zero beat indicator, representing thatthe continuous tuner 5 has tuned the oscillator 4 to 3128 kilocycles, isreceived by the counting stepper 9, the counting stepper continues itsstepping operation.` The counting stepper steps past the deenergized 64kilocycle line and stops at the 32 kilocycle output precision frequencygenerator line. The counting stepper then operates to close the circuitswitch 11 in the 32 kkilocycle line to connectV the 32 kilocycleprecision frequency generator with the mixer 6. The mixer 6 adds the128'kilocycles and 32 kilocycles to the 3000 kilocycles output of thebaud pass oscillator 7 and compares the 3160 kilocycle sum with theoutput frequency of the oscillator 4.` When the continuous tuner 5 tunesthe oscillator to 3160 kilocycles the mixer output operates the zerobeat indicator 8 to produce a beat or pulse which indicates to thecounting stepper that the oscillator 4 is tuned to 3160 kilocycles.

When the pulse produced by the zero beat indicator, representing thatthe continuous tuner 5 has tuned the oscillator 4 to 3160 kilocycles, isreceived by the counting stepper 9, the counting stepper continues itsstepping operation. The counting stepper steps past the deenergized 16kilocycle line and stops at the 8 kilocycle `output precision frequencygenerator line. The counting stepper then operates to close the circuitswitch 11 in the 8 klocycle line to connect the 8 kilocycle precisionfrequency generator with the mixer 6. The mixer 6 adds the 128kilocycles, the 32 kilocycles and the 8 kilocycles to the 3000 kilocycleoutput of the band pass oscillator 7 and compares the 3168 kilocycle sumwith the output frequency of the oscillator 4. When the continuous tuner5 tunes the oscillator 4 to 3168 kilocycles the mixer output operatesthe Zero beat indicator 8 to produce a beat or pulse which indicates tothe counting stepper that the oscillator 4 is tuned to 3168 kilocycles.

When the pulse produced by the zero beat indicator, representing thatthe continuous tuner 5 has tuned the oscillator 4 to 3168 kilocycles, isreceived by the counting stepper 9, the counting stepper continues itsstepping operation. The counting stepper steps past the deenergized4, 2and l kilocycle lines until it reaches the end ofthe travel. Thecounting stepper 9 discovers that the ordered tuning frequency of 3168kilocycles had been reached when it finds that there are no longer anyenergized precision frequency generator lines awaiting its action. Thecounting stepper 9 then indicates to the continuous tuner 5 that the endof the operation has been' reached by stopping the continuous tuner 5precisely at theiexact ordered tuning frequency of 3168 kilocycles.

v Whenthel oscillator 4 is tuned at 3168 kilocycles the inputiswitch 10opens and the system is ready to receive new ordered `tuning frequencyintelligence.

.Thel frequency data input switching unit 1 maybe replaced bya' binarykeyboard, which is manually operated .by an operator who knows what theordered tuning`freque`ncy of the oscillator 4 is and who depresses theproperkeys to energize those precision frequency generators whichtogether produce a frequency equal to the difference `between theordered tuning frequency and the lower frequency ofthe preselected bandof the ordered tuningfrequency. The operator also depresses a key toinitiate the operation of the band switch 2 to locate thegiband passoscillator 7 and the master oscillator 4 in the Vordered tuningfrequencys preselected band.

'I'h'e'binary system, described as a preferred embodimentiofthepresentinvention, is not essential for the proper 'operation of thepresent invention. Y If other than thesbinary system'isutilized, theinput intelligence, containing the yordered tuning frequency data, isfed toa' present invention are possible in the light of the aboveVteachings. lt is therefore to be understood that within the scope of theappended claim the invention may be` practiced otherwise than asspecifically described.

I claim: Y In combination; a tunable oscillator for generating .anyfrequency over a particular range of frequency,

said tunable oscillator including first means for dividing Vthe range offrequency that may be generated by saidi tunable oscillator intoadjacent Vfrequency bands, and

second means in Vsaid tunable oscillator that is contin-` uouslyadjustable over a range, said first means being variously connectible tosaid second means'toV afford the selected one of said frequency bandsand said second means being continuously adjustable for tuning saidtunable oscillator from end-to-end of the frequency band selected by theparticular connection of said first means to said second means; a motormeans connected to said second means for driving said second meansthrough its continuously adjustable range; a first precision oscillatormeans including a plurality of frequency determining means equal innumber to the number of said frequency bands and that are separatelyconnectible in said precision Voscillator to enable said precisionoscil-V lator to generate one frequency for Yeach of the frequency bandsrespectively of said tunable oscillator such that each frequencygenerated by said first precisionY oscillator means is slightly higherthan the lowest frequency of the corresponding band of said tunableoscillator; a band switch connected to said first meansV of said tunableoscillator and said plurality of frequency determining means of saidfirst precision oscillator means whereby each frequency band of saidtunable oscillator and the corresponding frequency of said firstprecisionoscillator means are activated concurrently; a plurality ofdistinct precision oscillator means for generating av open-closed switchmeans equal in number to said plurality of distinct precision oscillatormeans connected between said mixer and each of said plurality ofprecision oscillator means respectively; stepping means that progressesone step in response to each current impulse thereto, said steppingmeans being mechanically coupled to said last-mentioned switch means forclosing said plurality of switch means in sequence, said stepping meanshaving one step position in which said last-A mentioned switch Vmeansare all open and another step position in which said last-mentionedswitch means arey all closed and in stepping from its one step positionto its other step position in response to impulses, it passes throughall intermediate step positions and successive ones of saidlast-mentioned switch means are actuated from open to closed condition,said lastmentioned switch being coupled to said stepping means in asequence such that the one of said last-mentioned switch meansconnectedto the Vone of said pluralityof precision oscillator meansAwhich provides the highest' Among other applications, oscillator 4 mayof the binary frequencies is connected to said stepping means inposition to be closed first by said stepping means and the one of saidlast-mentioned switch means connected to the one of said plurality ofprecision oscillators which provides the lowest of the binaryfrequencies is connected to said stepping means in position to be closedlast by said stepping means, for coupling into said mixer from saidplurality of precision oscillator means the highest frequency rst downto the lowest frequency last; a zero beat means connected between saidmixer and said stepping means for providing a current impulse to saidstepping means each time the frequency of said l) tunable oscillator israised to where it equals the sum of the frequencies generated by saidrst precision oscillater means and said latter precision oscillatormeans that are switch-connected to said mixer; and manually operableswitch means connected to said motor for activating said motor.

References Cited in the le of this patent UNITED STATES PATENTS

